The present invention relates to the field of two-way wireless communication systems and more specifically to methods and apparatus for wireless communications in mobile telephone systems.
Conventional Cellular Systems
Cellular mobile telephone systems have developed due to a large demand for mobile services. Cellular systems "reuse" frequency within a number of cells to provide wireless two-way radio frequency (RF) communication to large numbers of users (mobile stations). Each cell covers a small geographic area and collectively groups of adjacent cells cover a larger geographic region. Each cell has a fraction of the total amount of RF spectrum which is available to support cellular users located in the cell. Cells are of different sizes (macro-cell or micro-cell) and are generally limited to a fixed capacity. The shapes and sizes of cells are functions of the terrain, the man-made environment, the quality of communication and user capacity. Cells are connected to each other via land lines or microwave links and to the public-switched telephone network (PSTN) through telephone switches. The switches provide for the hand-off of users from cell to cell and thus from frequency to frequency as mobile users move between cells.
Base Station (BTS)
In conventional cellular systems, base stations, or base transceiver stations (BTS), are the interface between mobile stations and the rest of the communications system. A base station is usually located in the center of a cell. The transmitting power of a base station determines the cell size. A base station typically has between one and sixteen transceivers where each transceiver uses separate RF channels. Base stations have RF transmitters and RF receivers co-sited for transmitting and receiving communications to and from cellular users (mobile stations) in the cell. The base stations employ forward RF frequency bands (forward carriers) to transmit forward channel communications to users and employ reverse RF bands (reverse carriers) to receive reverse channel communications from users in the cell. Conventional forward channel communications are static in that they employ fixed power, at fixed frequencies and have fixed sectors if sectorized antennae are used.
The forward and reverse channel communications use separate frequency bands so that simultaneous transmissions in both directions are possible. This operation is referred to as frequency domain duplex (FDD) signaling. Although time domain duplex (TDD) signaling, in which the forward and reverse channels take turns using the same frequency band is possible, such operation is not part of any widespread current cellular implementation.
The base station in addition to providing RF connectivity to users also provides connectivity to a Mobile Telephone Switching Office (MTSO). In a typical cellular system, one or more MTSO's will be used over the coverage region. Each MTSO can service a number of base stations and associated cells in the cellular system and supports switching operations for routing calls between other systems (such as the PSTN) and the cellular system or for routing calls within the cellular system.
Base Station Controllers
In conventional cellular systems, base station controllers (BSC) monitor and control one or more base stations. The number of base stations controlled typically is between several tens and several hundreds. The principal tasks of the base station controllers are frequency administration, the control of a base station, and exchange functions. The base station controllers assign RF carriers to support calls, coordinate the handoff of mobile users between base stations, and monitor and report on the status of base stations. The base station controllers can be located at the same site as the base stations or at a different site. Base station controllers and base stations together form a functional unit referred to as the base station subsystem (BSS).
Mobile Services Switching Center
The mobile services switching center (MSC) is the interface between the cellular system and the PSTN. The MSC is a switching exchange (switch) for routing calls from the fixed PSTN network through the base station controllers (BSC) and the base stations (BTS) to individual mobile stations (MS). The MSC switch provides the network with data about individual mobile stations. Depending on the cellular network size, one or more interfaces to the fixed PSTN network may exist through one or more switches. The number of base stations controlled by a single MSC depends upon the traffic at each base station, the cost of interconnection between the MSC and the base stations, the topology of the service area and other similar factors.
Operation and Maintenance Center
The operation and maintenance center (OMC) has access to both the MSC switches and the base station controllers in order to process error messages coming from the network and to control the traffic load of the BSC controllers and the BTS base stations. The OMC configures the BTS base stations through the BSC and allows components of the system to be checked.
A handoff between base stations occurs, for example, when a mobile user travels from a first cell to an adjacent second cell. Handoffs also occur to relieve the load on a base station that has exhausted its traffic-carrying capacity or where poor quality communication is occurring. The handoff is a communication transfer for a particular user from the base station for the first cell to the base station for the second cell. During the handoff in conventional cellular systems, there is a transfer period of time during which the forward and reverse communications to the mobile user are severed with the base station for the first cell and are not established with the second cell. A typical conventional cellular system has the transfer period designed to be less than 100 milliseconds.
Conventional cellular implementations employ one of several techniques to reuse RF bandwidth from cell to cell over the cellular domain. The power received from a radio signal diminishes as the distance between transmitter and receiver increases. All of the conventional frequency reuse techniques rely upon power fading to implement reuse plans. In a frequency division multiple access (FDMA) system, a communications channel consists of an assigned particular frequency and bandwidth (carrier) for continuous transmission. If a carrier is in use in a given cell, it can only be reused in cells sufficiently separated from the given cell so that the reuse site signals do not significantly interfere on the carrier in the given cell. The determination of how far away reuse sites must be and of what constitutes significant interference are implementation-specific details. The cellular Advanced Mobile Phone System (AMPS) currently in use in the United States employs FDMA communications between base stations and mobile cellular telephones.
In time division multiple access (TDMA) systems, multiple channels are defined using the same carrier. The separate channels each transmit discontinuously in bursts which are timed so as not to interfere with the other channels on that carrier. Typically, TDMA implementations also employ FDMA techniques. Carriers are reused from cell to cell in an FDMA scheme, and on each carrier, several channels are defined using TDMA methods.
In code division multiple access (CDMA) systems, multiple channels are defined using the same carrier and with simultaneous broadcasting. The transmissions employ coding schemes such that to a given channel on a given carrier, the power from all other channels on that carrier appears to be noise evenly distributed across the entire carrier bandwidth. One carrier may support many channels and carriers may be reused in every cell.
In space division multiple access (SDMA) systems, one carrier is reused several times over a cellular domain by use of adaptive or spot beam-forming antennae for either terrestrial or space-based transmitters.
TDMA Conventional Cellular Architectures
In TDMA systems, time is divided into time slots of a specified duration. Time slots are grouped into flames, and the homologous time slots in each frame are assigned to the same channel. It is common practice to refer to the set of homologous time slots over all frames as a time slot. Each logical channel is assigned a time slot or slots on a common carrier band. The radio transmissions carrying the communications over each logical channel are thus discontinuous. The radio transmitter is off during the time slots not allocated to it.
Each separate radio transmission, which should occupy a single time slot, is called a burst. Each TDMA implementation defines one or more burst structures. Typically, there are at least two burst structures, namely, a first one for the initial access and synchronization of a user to the system, and a second one for routine communications once a user has been synchronized. Strict timing must be maintained in TDMA systems to prevent the bursts comprising one logical channel from interfering with the bursts comprising other logical channels in the adjacent time slots. When bursts do not interfere, they are said to be isolated.
The isolation of one burst from the preceding and following bursts is crucial for TDMA systems. The defined burst structures are constructed to assist in the isolation process. A burst theoretically cannot completely fill its allotted time slot because radio transmitters neither commence nor cease transmitting instantaneously. TDMA implementations therefore allow time for radio signal strength to ramp up and to ramp down in each of the defined burst structures. During normal communications to and from a synchronized user, each burst does not quite fill its specified time slot. A guard period, T.sub.G, is inserted before or after each normal burst to allow for timing mismatches, multipath delays, and inaccuracies within the system. The initial synchronization bursts for accessing the system fill even less of a time slot than do normal bursts. The long guard period, T.sub.G, for synchronization bursts is used to overcome the timing mismatches caused by the unknown separation between a user and the base station.
Within a cell, the base station maintains a time base which users synchronize to during initial access. User synchronization to a particular base station is achieved using synchronization bursts sent periodically on a specific carrier by that base station and the reply synchronization bursts sent by the user. Those reply transmissions will arrive delayed at the given base station by the propagation time for radio signals over the separation between the user and the given base station. The separation is generally unknown because the users are mobile. Not only is a burst delayed, but in the cellular multi environment, multiple copies of the burst are received over some delay spread corresponding to multipath reception over reflected paths of varying lengths. A digital signal processing technique known as equalization is commonly used in RF communications to correct for multipath delay spreading and fading. After equalization, the base station can measure a single skewing delay time for the user synchronization burst. The base station then commands the user to correct for this delay time by time advancing the user bursts by an equal time interval. Thus each individual user has a time base set by the base station to ensure that the transmissions from al users will arrive back at the base station in synchronization with the base station time base.
These burst structures are detailed for two typical conventional cellular TDMA implementations. Under the European-defined "Global system for mobile communications" (GSM) standard, which is substantially copied in the United States within the PCS 1900 standard, each RF carrier occupies 200 kHz of bandwidth. Each carrier is divided into time slots of 577 .mu.s, organized into 8-slot frames lasting 4.615 ms. Each physical channel receives one time slot per frame, and a variety of logical channels may be constructed on a physical channel. The digital coding scheme used in GSM has a bit length of 3.69 .mu.s. A normal speech burst consists of 148 bits of information followed by 8.25 bits of guard time. Thus for GSM, the standard is T.sub.G =8.25 bits=30.44 .mu.s. The reverse channel synchronization (in GSM terminology, the random access) burst has 88 bits of signaling information followed by 68.25 bits of guard time. Thus for GSM, the T.sub.LG =68.25 bits=252 .mu.s.
Under the IS136 TDMA standard, each RF carrier occupies 30 kHz of bandwidth. Each carrier is divided into time slots of 6.67 ms, organized into 6-slot frames lasting 40 ms. Each logical channel receives two time slots per frame. The bit length for IS136 is 20.58 .mu.s. A normal reverse channel burst consists of 6 guard bits, 6 ramp bits, and 312 bits of mixed control signaling and data. Thus for IS136, T.sub.G =6 bits=123.48 is. The reverse channel synchronization burst has a longer guard period of 38 bits, so that T.sub.LG =38 bits=782.0 us for IS136.
In accordance with the above background, there is a need for improved wireless communication systems which overcome the limitations of conventional cellular systems.